1. Technical Field
The present invention relates to a wiring board for mounting a semiconductor element such as a semiconductor integrated circuit element.
2. Background
The conventional wiring board B for mounting the semiconductor element S′ will be described with reference to FIG. 3. The wiring board B includes the insulating board 30 where a plurality of insulating layers 33 having a via hole 34 are laminated on the upper and lower surfaces of the insulating plate 31 having a through-hole 32. Such a wiring board is described, for example, in Japanese Unexamined Patent Application Publication No. 2014-82393.
A plurality of conductors 35a and 35b are arranged on the surface of and inside the insulating board 30. The conductor 35a is a conductor for a differential signal. The conductor 35b is a conductor for grounding or a power supply. Through-conductors 36a and 36b are deposited in the through-hole 32. The through-conductor 36a is a through-conductor for a differential signal. The through-conductor 36b is a through-conductor for grounding or a power supply.
Via-hole conductors 37a and 37b are filled in the via hole 34. The via-hole conductor 37a is a via-hole conductor for a differential signal. The via-hole conductor 37b is a via-hole conductor for grounding or a power supply.
A plurality of semiconductor element connection pads 38a and 38b are formed in the central portion of the upper surface of the insulating board 30. The semiconductor element connection pad 38a is a semiconductor element connection pad for a differential signal. The semiconductor element connection pad 38b is a semiconductor element connection pad for grounding or a power supply. The semiconductor element connection pads 38a and 38b are electrically connected to the electrodes of the semiconductor element S′ through the solder.
External connection pads 39a and 39b are formed on the lower surface of the insulating board 30. The external connection pad 39a is an external connection pad for a differential signal. The external connection pad 39b is an external connection pad for grounding or a power supply. The external connection pads 39a and 39b are connected to the wiring conductors of the external electric circuit board through the solder.
Furthermore, a solder resist layer 40 is deposited on the surfaces of the insulating layer 33 and the conductors 35a and 35b of the outermost layer. The solder resist layer 40 on the upper surface side includes the opening exposing the central portion of the semiconductor element connection pads 38a and 38b. The solder resist layer 40 on the lower surface side includes the opening exposing the central portion of the external connection pads 39a and 39b. 
The wiring board B includes a transmission path for a differential signal and a transmission path for grounding or a power supply. The transmission path for a differential signal connects the semiconductor element connection pad 38a and the external connection pad 39a to each other through the conductor 35a, the through-conductor 36a, and the via-hole conductor 37a. In the transmission path for a differential signal, a pair of transmission paths are disposed in such a manner as to be mutually adjacent. The through-conductor 36a and the via-hole conductor 37a are connected to the central portion of the external connection pad 39a. On the other hand, the transmission path for grounding or a power supply connects the semiconductor element connection pad 38b and the external connection pad 39b to each other through the conductor 35b, the through-conductor 36b, and the via-hole conductor 37b. 
The differential signal is a transmission form having low electrical loss in the high-frequency transmission, and is useful as the form of transmitting a high-frequency signal. By the way, the wiring board B is designed so that its characteristic impedance comes as close as possible to 100Ω in the transmission path for a differential signal. It is preferred that the value of the impedance in each portion of the conductor 35a, the through-conductor 36a, or the via-hole conductor 37a constituting the transmission path for a differential signal is as close as possible to 100Ω, and that the difference in the impedance values in the transmission line is small. Thereby, the reflection of the signal is reduced, and the high-frequency signal can be transmitted efficiently.
In the wiring board B, the diameter of the external connection pads 39a and 39b is typically set to about 640 μm. The arrangement pitch between the adjacent external connection pads 39a and 39b is typically set to about 1 mm. The diameter of the external connection pads 39a and 39b is set as about 640 μm, and the connection area between the external connection pad and the wiring conductor of the external electric circuit board is increased, whereby the connection reliability between the wiring board B and the external electric circuit board is improved. The arrangement pitch between the external connection pads 39a and 39b is set as large as about 1 mm, whereby the electrical insulation reliability between the mutually adjacent external connection pads 39a and 39b is secured.
However, in the conventional wiring board B, the diameter of the external connection pads 39a and 39b is as large as about 640 μm, and therefore the electrostatic capacitance between the adjacent external connection pads 39a for a differential signal increases. For this reason, there is a tendency that the value of the impedance is less than 100Ω in the portion of these external connection pads 39a for a differential signal.
On the contrary, in the through-conductors 36a connected to the central portion of the external connection pads 39a for a differential signal, the arrangement pitch is as large as about 1 mm, and therefore there is a tendency that the electrostatic capacitance between the adjacent through-conductors 36a for a differential signal is reduced, and that the value of the impedance is greater than 100Ω. For this reason, there are problems that the impedance mismatch occurs, the reflection of the signal increases, and the high-frequency signal cannot be efficiently transmitted between the external connection pad 39a for a differential signal and the through-conductor 36a. 